Love you 7-zip Beta or stable Everything is the same for me 7-zip is the best for compression though not for the UI which ranks below Winrar and Winzip
Maybe the latest alpha does the job? It's an unstable release but I haven't had any problems using it. Or do you mean the windows 8 install.wim? Maybe the wim format has had an update. I'll check if the windows 7 dism can handle it.
In the spirit of open source software, the author of "Easy 7-zip" took an app that hadn't been updated in 3 years (7-zip), and added features that they thought made it better. More options are a good thing.